1. Field of the Invention
The present invention belongs to a technical field for measuring luminance of a liquid crystal display.
2. Description of the Related Art
As one of diagnostic methods in a medical field, a diagnosis using a diagnostic image taken by a medical measurement apparatus such as an ultrasonic diagnostic apparatus, a CT diagnostic apparatus, an MRI diagnostic apparatus, an X-ray diagnostic apparatus, or an FCR (Fuji computed radiography) is conducted. The diagnostic image is generally reproduced on a photo film. In diagnosis, the photo film is observed by setting it in Schaukasten. In addition, so-called monitor diagnosis in which the diagnostic image is managed as digital image data, and displayed on a display (electronic Schaukasten) to observe it in diagnosis comes into wide use to become general for convenience of image data management.
In such a diagnostic image, a slight density difference thereof is used as a diagnostic basis. Thus, it is very important to always display the image with adequate luminance and gray scale.
When a diagnostic image is recorded in a photo film, the gradation of the luminance (gray scale) of the image is stored in a state in which it is recorded in the photo film. Thus, during a long period for which the film is stored, an image with substantially the same quality is obtained in every observation. In contrast to this, in the monitor diagnosis, the diagnostic image is stored as digital image data and treated. Thus, when the image is displayed On a display in observation, the gradation of the luminance of the displayed image is greatly changed according to a display performance of the display to be used. In addition, even when the same display is used, there is a case where the display performance is varied by deterioration with time or the like so that image display of the same quality cannot be kept.
Therefore, when the monitor diagnosis is conducted, the check of the performance of a display by a user in advance, that is, the quality control of the display is required.
In particular, it is important to measure a luminance characteristic in order to conduct display with an adequate gradation of luminance. The luminance characteristic measurement is conducted by, for example, measuring images with various luminance levels which are displayed on the display using a luminance meter.
With respect to the luminance meter used for luminance measurement of the display, there are a non-contact type telescopic luminance meter for conducting measurement in a position distanced from the display by a predetermined distance and a contact type luminance meter for conducting measurement in contact with the display surface of the display.
The telescopic luminance meter has an optical system in the luminance meter and condenses light from a range limited by a lens. Thus, a measurement field of view can be narrowed so that it is limited to narrowness of, for example, 1 degree. On the other hand, the contact type luminance meter is attached onto the display surface of the display by a sucking disc or the like and directly receives light from the display. A measurement field of view is very wide.
In view of such a difference of structure, the telescopic luminance meter has high precision but is expensive. In contrast to this, the contact type luminance meter is commercially available at relatively low cost. Therefore, the contact type luminance meter is used for general luminance measurement of a CRT (cathode-ray tube).
In the case where display luminance of the display is measured using the above two types of luminance meters, when it is a CRT, a view angle characteristic (view angle dependence) of the CRT is small. Thus, even in the contact type luminance meter, substantially the same measurement value as the telescopic luminance meter is obtained.
However, with respect to an LCD (liquid crystal display), in view of its structure, there is a view angle characteristic, that is, a characteristic in which it appears to change display luminance and display chromaticity according to an angle at which a display screen is viewed, such as a characteristic in which it appears to be light by light leakage at an oblique view even when it is black at a front view. Thus, when measurement is conducted by the contact type luminance meter generally used for the CRT, accurate measurement cannot be conducted resulting from of a wide measurement angle of the luminance meter and the view angle characteristic of the LCD. In particular, an error in a low luminance side becomes larger due to light leakage in a black level.
FIGS. 4A and 4B show comparisons of results obtained by measuring luminance of the LCD and that of the CRT using the contact type luminance meter and the telescopic luminance meter. The abscissa indicates a measurement value in the telescopic luminance meter and the ordinate indicates a ratio of a measurement value in the contact type luminance meter to the measurement value in the telescopic luminance meter. As shown in FIG. 4B, in the case of CRT, the measurement value in the contact type luminance meter coincides with the measurement value in the telescopic luminance meter over a wide range of luminance. However, in the case of the LCD as shown in FIG. 4A, the measurement value in the contact type luminance meter is greatly shifted on a low luminance side.
Further, in the LCD, liquid crystal molecules are aligned between two electrode substrates. Thus, when a pressure is applied onto the display surface by a touch or the like, an interval between the electrode substrates is changed so that display luminance is changed. Therefore, when the contact type luminance meter is fixed to the display surface of the LCD by means of a sucking disc or the like so that the weight of the luminance meter is supported on the display surface, there is also a problem in that a suitable measurement value cannot be obtained.
As described above, in the case of the telescopic luminance meter, a suitable measurement value can be obtained even in measurement of the LCD as in the CRT. However, the telescopic luminance meter is very expensive because of the precision structure of its optical system. Thus, when the meter is used as a luminance measuring tool by a display user as in a medical institution, a burden thereto is large, which is not suitable.
An object of the present invention is to solve the above conventional problems, and therefore to provide a luminance measuring device for an LCD which can accurately measure display luminance of the LCD by an inexpensive contact type luminance meter so that, for example, a suitable calibration can be stably conducted.
In order to attain the object described above, the present invention provides a luminance measuring device for a liquid crystal display comprising:
photometric means including: a contact type luminance meter; a light shielding cushion member surrounding a vicinity of a light receiving portion of the contact type luminance meter; and holding means for fixing the contact type luminance meter to the liquid crystal display such that the light shielding cushion member lightly presses onto a display surface of the liquid crystal display and a relationship between an orientation of a light receiving portion of the contact type luminance meter and an orientation of the liquid crystal display is kept constant;
converting means for converting a first luminance measurement result by the contact type luminance meter into a second luminance measurement result corresponding to a telescopic luminance meter, the converting means previously being prepared from the first luminance measurement result by the contact type luminance meter and the second luminance measurement result by the telescopic luminance meter; and
processing means for conducting conversion processing using the converting means to a luminance measurement result of the liquid crystal display by the photometric means.
Preferably, the converting means is set for each kind of liquid crystal display, and the processing means selects the converting means corresponding to one kind of liquid crystal display as a measurement subject and conducts the conversion processing.